The present disclosure relates to semiconductor switches for switching, controlling and/or directing electromagnetic radiation, particularly microwave radiation in the portion of the spectrum known as “millimeter waves.” Such millimeter wave radiation typically is employed in many radar applications, particularly collision avoidance radar used in various types of vehicles and craft.
Semiconductor microwave switches are known in the art, and have been employed in such applications as steerable or directional antennas, especially dielectric waveguide antennas used to send and receive steerable millimeter wave beams in various types of radar devices, such as collision avoidance radars. In such antennas, semiconductor switches may be employed to provide an antenna element with an evanescent coupling edge having a selectively variable coupling geometry. The coupling edge is placed substantially parallel and closely adjacent to a transmission line, such as a dielectric waveguide. As a result of evanescent coupling between the transmission line and the antenna element when an electromagnetic signal is transmitted through the transmission line, electromagnetic radiation is transmitted or received by the antenna. The shape and direction of the transmitted or received beam are determined by the selected coupling geometry of the evanescent coupling edge, as determined, in turn, by the pattern of electrical connections that is selected for the edge features of the coupling edge. Semiconductor switches may be employed in the antenna element as one mechanism for varying this pattern of electrical connections. See, for example. U.S. Pat. No. 7,151,499 (commonly assigned to the assignee of the present application), the disclosure of which patent is incorporated herein by reference in its entirety.
A typical prior art semiconductor microwave switch array 10 that may be used in an antenna of the aforementioned type is shown in FIG. 1. The prior art switch array 10 is formed on a wafer or substrate 12 of semiconductor material (e.g., Si, Ge, or GaAs) by forming a plurality of PIN junctions, each comprising a P-doped region that serves as a P-type electrode 14, an N-doped region that serves as an N-type electrode 16, and an insulative substrate gap 18 between the two electrodes 14, 16. Successive PIN junctions are separated by an insulative substrate region 20, and the successive PIN junctions are of alternating polarity (i.e. P-I-N, alternating with N-I-P), as shown in FIG. 1. The surface of the substrate 12 is covered by a thin passivation layer, which may be a suitable insulative material, such as SiO2 or Si3N4, for example. The passivation layer is subjected to a first photolithography process to form a linear passivation region 22 overlying each of the insulative substrate gaps 18. A metal layer (e.g., Ag, Al, Au, Cu, Pt) is then formed or deposited over the surface of the substrate 12 and over the passivation regions 22 by any suitable conventional process (e.g. electroplating or electrodeposition). The metal layer is then subjected to a second photolithography process to form an array of first contacts 24 and an array of second contacts 26, wherein each of the first contacts 24 is separated from its adjacent second contact 26 by an exposed passivation region 22. Each of the first contacts 24 thus connects the P-type electrodes 14 in each adjacent pair of PIN junctions, while each of the second contacts 26 connects the N-type electrodes 16 in each adjacent pair of PIN junctions.
Each of the PIN junctions provides a switch having an “open” state when no potential is applied across the junction, and a “closed” state when a potential above a predefined threshold potential is applied across the junction. When a switch is open, the exposed passivation region 22 provides a “slotline” through which electromagnetic radiation of suitable wavelength may be directed. When a suitable potential is applied across the PIN junction, the switch is closed, and an electron-hole plasma (not shown) is created and injected into the passivation region 22 between the electrodes 14 16, thereby shorting the electrodes. This plasma reflects the electromagnetic radiation, effectively blocking the path of the radiation through the slotline provided by the passivation region 22.
One disadvantage of the prior art semiconductor switch array 10, as described above, is that the plasma created by the application of the potential across the electrodes of each PIN junction switch is not effectively confined to the area in the vicinity of that switch. Thus, in the switch array 10, the plasma created by each PIN junction switch tends to diffuse across the surface of the substrate 12, so that it may “contaminate” other switches and slotlines in the array thereby degrading the performance of those switches and slotlines, and compromising the functioning of the array as a whole. Moreover, within each switch, the plasma tends to diffuse along the length of the slotline, away from the electrodes, thereby degrading the performance of the slotline controlled by that switch.
Thus, it would be a significant improvement in the state of the art to provide a semiconductor microwave switch in which the effects of plasma diffusion are minimized, without compromising the overall performance of the switch or of any array of which the switch forms a part. It would be a further advantage to provide such a switch without substantially increasing the cost of manufacture of the switch or the switch array.